1. Field of the Invention
The present invention relates to a semiconductor device, and more particularly to a semiconductor device which works as a semiconductor sensor.
2. Description of the Background Art
FIG. 27 is a cross section showing a structure of a semiconductor device 90, as an example of the background-art semiconductor sensor, comprising moving electrodes such as an acceleration sensor, a pressure sensor and an angular velocity sensor. In general, the semiconductor sensor with the moving electrodes has a structure in which the moving electrode is contained in a hollow cavity. In FIG. 27, a moving part MV provided on a main surface of a sensor substrate SC is covered with a hollow cavity CV (hereinafter, referred to as a cap).
The sensor substrate SC has at least a semiconductor substrate (not shown), the moving part MV and a semiconductor element (not shown) for generating an electrical signal according to the displacement of the moving part MV, and the electrical signal generated by the semiconductor element is transmitted to a signal processing substrate AS for performing a predetermined signal processing through a wiring interconnection WR. The signal processing substrate AS has a structure of application specific IC (ASIC) to perform a function of calculating acceleration, pressure and the like on the basis of the electrical signal given from the sensor substrate SC.
The sensor substrate SC and the signal processing substrate As are mounted on a die pad DP of a lead frame and the signal processing substrate AS is electrically connected to an inner lead IL through the wiring interconnection WR. The sensor substrate SC and the signal processing substrate AS are sealed by a mold resin MR together with the die pad DP and the inner lead IL, whereby a resin-sealed package is achieved.
FIG. 28 is a perspective view showing a state before the semiconductor device 90 is sealed by the resin. As shown in FIGS. 27 and 28, the die pad DP in the semiconductor device 90 has such a depressed die pad structure as to be lower in position than the inner lead IL. This structure makes the inner lead IL and the surface of the semiconductor substrate such as the signal processing substrate As on the die pad DP almost flush in height, thereby facilitating wire bonding.
The cross section taken along the line A--A of FIG. 28 corresponds to the structure view shown in FIG. 27.
Further, FIG. 29 is a conceptional view showing a structure of the moving part MV in the sensor substrate SC. As shown in FIG. 29, the moving part MV has a moving electrode MVP which is movably supported by a beam BM and a fixed electrode FXP which is so provided as to have a clearance between the moving electrode MVP and itself and form static capacitance therebetween.
The fixed electrode FXP is an electrode of semiconductor element having a structure to detect variation of static capacitance generated by the displacement of the moving electrode MVP. The cross section taken along the line B--B of FIG. 29 is shown in FIG. 30.
Thus, the sensor substrate SC needs to ensure the movement of the moving electrode MVP and therefore the moving part MV is covered with a cap CV made of silicon or glass to prevent the mold resin MR from entering.
Providing the cap, however, causes problems of an increase in manufacturing cost due to not only an increase in the number of parts but also necessity of a process to bond the cap CV on the sensor substrate SC.